Winter Gardening Without Heated Greenhouses

Greenhouse covered in snow

It might seem like a myth that you can grow food in an unheated greenhouse during the winter, but we’re here to prove you wrong. As long as there is sunlight, there are several techniques you can use to keep your crops warm enough to harvest well into the cold weather. The key? Layers. 

Just as you bundle up to go outside during the cold winter months, adding layers to your greenhouse can provide ample insulation to protect against snow and numbing temperatures. By implementing a few new strategies and figuring out the best food to grow in winter, you can create your own “cold” house!

The following is an excerpt from The Winter Harvest Handbook by Eliot Coleman. It has been adapted for the web.


Over the course of devising, developing, and improving our winter-harvest practices, we have amassed a collection of technical studies on hardy crops and the effect of freezing temperatures. Copies of research papers on all aspects of greenhouse growing fill our file cabinets.

Hardy Crops

In the natural world, hardy crops like spinach and chard inhabit niches where resistance to cold is a requirement for survival. Winter-annual crops, like mâche and claytonia, have found their space to grow by germinating in fall, growing over winter, and going to seed in spring. Whereas the outdoor winter climate here in Zone 5 Maine is too harsh for even the hardiest of these crops, the twice-tempered climate under the inner layer of our cold houses offers them conditions within the range to which they are adapted.

Even after working with this unheated system for many years, I continue to be amazed by the daily miracle. The same three words keep coming to my mind every winter day—unheated, uninsulated, unbelievable! When you enter the protection of one of our cold greenhouses, you can take off your parka because the microclimate you encounter is that of a location approximately one and one-half USDA zones to the south. When you reach your hand under the row covers you have moved another one and one half zones south where the Maine winter definitely does not prevail. Outdoors the climate is Zone 5; under the inner layer, the climate is Zone 8.

Plants growing in a greenhouseWe started using the phrase cold house to describe these structures because the word unheated made it sound as if we are not doing something—heating—that we should be doing. Furthermore, it may be clearer to use the descriptive phrase high tunnel or cold tunnel and avoid the word greenhouse altogether since many people assume that greenhouses, if unheated, are expensive super, insulated technological marvels or complicated heat-storage devices. Ours are neither.

The best short statement to describe our approach is the epigraph to this chapter by Buckminster Fuller from his book Shelter (1932)—“Don’t fight forces; use them.” Instead of bemoaning the forces of winter and trying to fight them head on, we have limited our intervention to the climatic protection provided by two translucent layers. Instead of the usual thinking, which only sees greenhouses as a way to grow heat-loving crops during cold weather, we have said, “So it’s cold. Great! What vegetables thrive in the cold?” The answer is some thirty or so hardy vegetables.

Fighting force requires energy, and energy costs money. Our cold-house approach takes advantage of everything our two translucent protective layers can get for free from the sun as well as the residual heat of the soil mass and then works within those limits. The same applies in reverse during the summer. When the protected microclimate inside the houses turns warm, we don’t fight that warmth with motorized greenhouse cooling systems. We use it to grow heat-loving crops.

The Outer Covering

When we first started growing crops in cold houses, we covered all of the houses with just a single layer of plastic. We made that choice to maximize light input. Using two layers of plastic and blowing air into the space between them to inflate the plastic provides more protection from cold, but it also cuts out an additional 10 percent of available light. Also we prefer to work with systems that are inexpensive and simple. Thus, we decided to forgo the expense of the second layer and the electric blower required to inflate the layers.

We are interested in comparing greenhouse plastic from different manufacturers to find the type of cover that lets in the most light and keeps in the most heat. In our cold climate we want to increase daytime heat gain and light levels, so we favor covers that maximize those inputs. Plastic covers are available with an anti-drip coating that causes condensed moisture to form a thin film instead of droplets. Covers with this type of coating not only let in more light but the thin film of moisture also acts to reflect back the heat waves radiating from the soil at night thus helping to keep the air inside the house warmer. Growers in the southern states where cold is not as intense may want to use plastics designed to block infrared input and thus help to keep the greenhouse from overheating.

Using Double Covers

For experimental purposes, we trialed one small air-inflated house (17 feet by 36 feet) without heat. The temperature records we kept show that nighttime low temperatures averaged 4˚ F (2.2˚ C) warmer in the air-inflated house than in a cold house with a single-layer outer covering. For example, on a cold night, when the low temperature was –8˚ F (–22˚ C) outside, the temperature dropped to 2˚ F (–17˚ C) inside a single-layer house and 20˚ F (–7˚ C) under the inner layer of row cover. By comparison, in the air-inflated house, the low temperature was 7˚ F (–14˚ C) and 24˚ F (–4° C) under the inner layer of row cover.

Our observations of crops during this trial showed some interesting comparisons between the two houses. Although we could detect no apparent difference in the quality of the crops of harvestable size, we did notice faster growth of new seedlings in the air-inflated house in winter. That house also warmed more quickly on cold mornings because the layer of sunlight-blocking frost that forms on the inside of the plastic melted off more slowly in the single-layer house. Based on this trial, we began double-covering the cold houses where we would be sowing new crops from December 15 to February 15. With the rest of the cold houses, such as one that protects leeks for midwinter harvest, we continue with our inclination in favor of simplicity and better light input and use only a single sheet of plastic to cover the house.

The Inner Layer

The success of our work with cold frames and then row covers convinced us of the Cloth cover for cropsbenefits of the inner-and outer-layer concept. We wondered if we could do even more. We thought about placing smaller tunnel greenhouses inside the larger ones as some Japanese farmers were doing, but, on further consideration, we decided that the management and ventilation seemed complicated and the use of space seemed inefficient. We considered motorized night-curtain systems of reflective material, which are sometimes used in heated greenhouses, but they were very expensive. After exploring all of the above, we reverted, as we usually do, to the simplest, least expensive option—a floating row cover as the inner layer. If we had started our winter operation with more elaborate systems, we never would have known if they were really necessary.

Although we worried that floating row covers might be considerably less protective against cold than glass cold frames, the self-ventilating nature of the row covers and their availability in large sizes were overwhelming advantages. And, further, we did not know if we had yet pushed our crops to the lowest temperatures they would tolerate in a protected microclimate.

Our opinion, after many years of practical experience with winter-harvest systems, is that the protected microclimate we have created is successful principally because it protects against wind (think of wind-chill readings and the desiccating effect of cold dry winds on winter vegetation) and, secondarily, because it protects against the fluctuating wet-dry, snow-ice conditions of the outside winter. In this microclimate, a few degrees of temperature one way or the other does not appear to be the crucial determinant of survival for most of our crops.

Easy Handling

We plan to put the row covers over the crops just before the weather gets cold enough to freeze inside the greenhouse. One of the delights of using row covers inside a greenhouse is the ease of management. Since there is no wind, there is no need to bury or weigh down the edges. Even large pieces can be removed and replaced easily for harvest and other access needs without worrying about them being caught by a gust of wind.

For the large houses our interior covers are 20 feet wide by 50 feet long, large enough to cover one quadrant of a 30-by-96 foot greenhouse. The 48-foot houses are covered by two pieces each 15 feet wide. The covers are supported, 12 inches above the soil, by flat-topped wire wickets. We make the wickets from 76-inch-long straight lengths of number 9 wire. The flat top is 30 inches wide, the same width as the beds, and each leg is 23 inches long. Thus, when the wickets are in place, they do not block the access path between the beds.

We space the wickets every 4 feet along the length of a bed, which provides sufficient framework to support the row-cover fabric. When the fabric is in place, we pull it taut and clip it to the end wickets of the quadrant with clothespins. That prevents the fabric from sagging under the weight of condensed moisture, which can be quite substantial. We have noticed occasional frost damage at points where the fabric has drooped down and frozen to the leaves below, as opposed to no damage when the fabric does not touch the plants. The edges of the fabric drape down over the edge of the wickets and rest along the side of the greenhouse or in the pathway.


Recommended Reads

Presenting the Four-Season Harvest

The Original Organic Pioneer – An Interview With Eliot Coleman

Read The Book

The Winter Harvest Handbook

Year Round Vegetable Production Using Deep-Organic Techniques and Unheated Greenhouses

$29.95

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